Kaolin crude originates from sedimentary deposits found in such locations as the state of Georgia in the United States and Brazil. Kaolin, containing largely the mineral kaolinite, is very widely used in paper coating and filling. In the United States, kaolin pigment production was nearly 7.5 million tons in 2003 generating an estimated revenue close to $1.6 billion. Upgrading the kaolin purity so that it can be used for producing premium brightness products is of particular importance. The paper market trend is moving towards producing thinner paper grades with the same properties of a thicker paper in terms of opacity and print fidelity resulting in two grades of kaolin which are growing at a fast rate. The two grades are (1) narrow particle engineered grade kaolin and (2) glossing grade kaolin both of which require premium brightness (90-91 GE Brightness). In order to achieve this high level of brightness it is necessary to remove colored impurity minerals such as titanium and iron based impurities from the kaolin.
There are several beneficiation techniques that are employed in the industry to remove the impurities, such as size classification methods, magnetic separation, flotation and selective flocculation. These beneficiation techniques, regarded as wet processes, remove colored titanium and iron based impurities. As a first step, kaolin crude is initially dispersed in water, degritted (process in which coarse particles and impurities greater than 37-44 micron are removed) and then the resulting slurry is subjected to beneficiation.
Titanium and iron impurities such as rutile, anatase, hematite and pyrite can sometimes possess weak magnetic susceptibility. In such cases High Intensity Magnetic Separation (HIMS) process can be used to remove the magnetic impurity particles. However, the method is not efficient enough to capture submicron sized magnetic particles and therefore limits the capability of magnetic separation in producing high brightness kaolin.
Froth flotation is another process for removing colored titaniferrous impurities. Here the discoloring impurities are selectively hydrophobized by the addition of a collector. Examples of such collectors include fatty acids such as tall oil fatty acid, alkyl sulfonates, alkyl hydroxamate (AHX) and mixtures thereof. After the addition of the collector, the slurry is conditioned or mixed prior to flotation. After conditioning the hydrophobized particles are levitated to the surface—the air/water interface—by means of air bubbles that are injected at the bottom of the flotation cell. U.S. patents that describe removing colored impurities by flotation include U.S. Pat. Nos. 3,979,282; 3,450,257; 4,472,271; 4,492,628; 4,629,556; 5,522,986; 5,685,899; and 5,810,998. A variation of froth flotation using carrier particles such as calcium carbonate is described in U.S. Pat. No. 2,990,958.
U.S. Pat. No. 4,871,466 describes a method of producing alkyl hydroxamic acid collectors and U.S. Pat. Nos. 4,629,556 and 6,378,703 each describe a froth flotation method for beneficiating kaolin using alkyl hydroxamic acids.
Selective flocculation is another well known beneficiation process. Normally, the flocculant is initially adsorbed onto and bridges between mineral particles of similar composition thus binding the particles together. The bonded particle aggregates then form larger aggregates or flocs which settle out of the suspending medium by gravity. Flocculants can be natural products such as starch, guar gum and alginates or synthetic polymers such as polyacrylamides, polyacrylates and polyethylene oxides. Selective flocculation is an effective process for recovering fine to ultrafine minerals that respond poorly to conventional beneficiation processes such as flotation and magnetic separation. The successful use of selective flocculation on mixtures of fine mineral particles such as kaolin clays, iron-bearing minerals, phosphates, potash, copper ores and coal is known in the industry.
Selective flocculation involving the activation of the impurity with polyvalent cations, as described in U.S. Pat. Nos. 3,371,988; 3,701,417; 3,837,482 and 3,862,027), conditioning with ammonium salt, as described in U.S. Pat. No. 4,604,369) or with fatty acid and polyvalent cations (U.S. Pat. No. 5,535,890) and then selectively flocculating the impurities with charged anionic water soluble polymers, usually polyacrylamides.
Specifically, U.S. Pat. No. 5,535,890 describes a selective flocculation process for purifying the kaolin clay containing a colored impurity which involves selective flocculation of the colored impurity as a dense dun colored thick mass separated as a lower layer and a white layer of purified kaolin that is low in colored impurities. The reagents used include typical dispersants such as sodium silicate, sodium polyacrylate, sodium hexameta phosphate, divalent metal ions, fatty acid and high molecular weight anionic polymer. The process suffers from low recoveries or yields.
U.S. Pat. Nos. 6,390,301 and 6,041,939 each describe a similar process for beneficiating kaolin by a selective flocculation process employing an alkyl hydroxamic acid and flocculating agent.
U.S. Pat. No. 6,200,377 describes an improved process for the beneficiation of kaolin crude that contains minerals that chelate with hydroxamates. The use of a silicon containing compound in combination with hydroxamate results in a more effective separation of minerals that chelate with hydroxamate. Also described is the addition of a silicon-containing compound to increase the interaction of the hydroxamates with the chelatable minerals, which results in a more effective beneficiation.
Selective flocculation has also been used to beneficiate other mixtures of mineral particles. Iron-bearing ores, specifically taconite, are commercially processed using selective flocculation. The iron ore is ground and then dispersed with caustic and sodium silicate. The dispersed ground ore is then selectively flocculated with a corn starch flocculant to separate hematite, an iron oxide mineral.
Phosphate minerals are beneficiated using selective flocculation to separate them from the associated clays, as described in U.S. Pat. No. 2,660,303. Potash is also beneficiated by selective flocculation using a anionic polyacrylamide flocculant and/or ethoxylated alkylamic alkylguanidine complex. Another selective flocculation process to beneficiate alkaline carbonate minerals, phosphate minerals, zeolites and bauxites is described by U.S. Pat. No. 5,535,890. In this process, fatty acids and polyvalent cations are used to recondition the mineral suspension. Another beneficiation process involves leaching the kaolin clay with iron-reducing reagents such as zinc or sodium hydrosulfite. This leaching method is however, limited to removing iron contaminants only. Other known leaching reagents and/or processes are not currently economical for removing titanium impurities.
Due to the limitations of these various beneficiation processes to separate certain minerals from mixtures of minerals, such as the discoloring impurities in kaolin clays, there is a need in the industry for a process that is more effective and efficient.